Among the original mainline US highways (those whose route numbers end in 0 or 1), US Highway 91 holds the unfortunate distinction of having its length cut the most over the years. When the American Association of Highway Officials (today’s AASHTO) created it in 1926, US 91 stretched over some 1,468 miles from Sweet Grass, Montana to Long Beach, California.

As it wound down through Montana from the Canadian border at Sweet Grass, US 91 passed through Great Falls and the state capital at Helena. The highway continued on to Butte and a junction with Interstate 90. US 91 spent another 125 miles in Montana and then crossed into Idaho.

Present-day US 91 begins at Idaho Falls and it parallels Interstate 15 closely until Pocatello, a city made famous by the Judy Garland song "Born In A Trunk". The highway veers away from I-15 as it enters Utah, following a section of the old Mormon Trail. The existing US 91 only gets as far as Brigham City, where it ends at an interchange with Interstates 15 and 84. The historic route, however, continued on, duplexed with US Highway 89, through Salt Lake City, Provo, and St. George.

Just a few miles south of St. George, US 91 turned west to pass through a small corner of Arizona, and entered Nevada at the city of Mesquite. In Nevada, US 91 cuts a diagonal through the southernmost point of the state on its way to the greater Las Vegas area. The famous "Las Vegas Strip" once carried US 91 through the city, with fabulous casinos on either side of the street.

Except for the portions in Idaho and Utah, Interstate 15 has totally replaced the US 91 designation. Some states, rather than build new roadway, used existing 91 pavements, completely obliterating the old road. The last vestiges of US 91 in California disappeared in July 1964 during the state’s highway renumbering, when it was converted to California state highway 91. With that truncation, the once-mighty US Highway 91 was reduced to a mere 163 miles.